1. Field of the Invention
The present invention relates to a nitride semiconductor laser made of a nitride semiconductor (InaAlbGa1-a-bN, 0≦a, 0≦b, a+b≦1), and particularly to a nitride semiconductor laser device formed with a good resonant surface that is reproducible by way of a cleavage process and has an excellent lifetime and mass productivity.
2. Discussion of the Related Art
Recently, small size, lightweight, high reliability, and a high output properties have been required for nitride semiconductor laser devices, and they are used as a light source for an optical disc such as a DVD and a light source for medical equipment. Various research has been conducted on nitride semiconductor laser devices.
For example, as a practical laser device, a nitride semiconductor laser device capable of continuous-wave oscillation of more than 10,000 hours at room temperature is described in Japan Journal of Applied Physics, Vol. 37 (1998), pp. L309-L312, Part 2, No. 3B, Mar. 15, 1998. According to this article, the nitride semiconductor laser device is obtained by the following procedures. First, a nitride semiconductor substrate made of GaN is selectively grown on a sapphire substrate. A plurality of nitride semiconductor layers comprising a laser device structure are stacked on the nitride semiconductor substrate. Then, the sapphire substrate is removed and a resonator is formed by a cleavage process. This nitride semiconductor laser device has a ridge stripe. The ridge stripe is formed by partially etching from a p-side contact layer made of p-GaN to a p-side cladding layer of a super lattice structure. The super lattice structure made of p-Al0.14Ga0.86N/GaN disposes a p-electrode on the stripe, and a resonance surface is formed by the cleavage process.
However, many defects may occur in a wafer of the laser device described above, such as poor reproducibility upon cleavage or a poor lifetime thought to be caused by deterioration due to insufficient heat dissipation. The cause is thought to be that even when the foreign substrate made of a different material from the nitride semiconductor layer is removed, the cleavage surfaces of the nitride semiconductor substrate and the nitride semiconductor are not completely the same. Therefore, chipping off and crushing occur in the top layer of the stripe ridge by the impact generated when the resonance surface is formed by cleavage. There have been other problems with the construction of the nitride semiconductor laser device. These problems include that the width of the ridge is small compared with the chip size of the nitride semiconductor laser device so that the damage tends to concentrate in the ridge. Also, the deterioration of the ridge progresses due to oscillation as a laser device even if the damage such as chipping off was not seen when cleaving, and consequently, the laser device is incapable of long hours of continuous-wave oscillation.
In addition, a large chip size is required when the p-electrode and the n-electrode are disposed on the same surface of a wafer. However, the chip size can be reduced by fabricating a laser device with a counter electrode structure where the n-electrode is disposed on the rear surface of the nitride semiconductor substrate. As a result, the number of chips formed in a wafer can be greatly increased. However, a nitride semiconductor laser device having the n-electrode with good ohmic features on the rear surface of the substrate has not been achieved.